1. The Field of the Invention
The present invention relates to x-ray devices. More particularly, the present invention relates to x-ray tubes having components constructed of a copper chromium alloy material for enhanced thermal management and thermal stability.
2. The Relevant Technology
X-ray devices are widely used in applications ranging from medical radiology to industrial diagnostics. A common problem encountered in the design and operation of x-ray tubes used in such devices relates to the management of the extremely high temperatures that are present. Heat management is particularly troublesome in the region of the rotating anode and rotor assembly. During operation, extreme temperatures are generated at the anode's focal track, which are then transferred to other parts of the rotor assembly. These temperatures can adversely affect the operating life of the x-ray tube. For instance, bearings that assist the rotating anode to rotate can fail, and other parts of the rotor assembly are prone to failure from the constant thermal expansion and contraction.
The problems related to high temperatures produced in the x-ray tube have been partly addressed by providing an emissive coating on the rotor portion of the x-ray tube assembly. Preferably, the coating possessed thermal characteristics that allowed components of the x-ray tube--such as the rotor--to operate more satisfactorily under the extreme operating temperatures. For example, in the past a thin, oxygen-deficient titanium oxide layer was applied onto the rotor skirt by a plasma spraying process. However, this coating has not been entirely satisfactory--especially over longer operating periods. In particular, the repeated thermal cycling of an x-ray tube structure tends to cause an emissive coating of this sort to flake or spall away from the rotor skirt. This debris can then contaminate other components within the x-ray tube, and lead to premature failure of the tube. Moreover, there often is a thermal mismatch between rotor material and the coating material, which tends to weaken the bond between the two materials as they thermally expand. Again, this leads to the undesired situation of the coating flaking or spalling and contaminating the x-ray tube.
Use of such coatings can also give rise to other problems. For instance, during the manufacturing of the x-ray tube device, difficulties are often encountered in getting the coating to properly adhere to the rotor substrate and/or the other x-ray components. To ensure proper adhesion typically requires an additional manufacturing step prepare the rotor substrate. For example, the rotor substrate may be "roughened" by blasting the substrate with a grit material. This process is undesirable for several reasons. First, the need for air extra manufacturing step adds cost and complexity to the overall manufacturing processes. Second, some of the grit material used in the roughening process invariably will become physically embedded within the rotor substrate material. This grit material can then shed from the rotor during operation of the x-ray tube, especially after repeated use. Again, release of such foreign matter within the sealed environment of an x-ray tube leads to contamination and premature failure of the tube.
As noted, other components within the x-ray tube are also subject to various problems associated with the high operating temperatures. For example, a bearing support structure is often connected to a "nose" portion of the rotor which is in turn connected to the rotating anode. The bearing support structure is typically disposed within the rotor sleeve portion and, due to its close proximity to the rotating anode, is also exposed to extreme temperature fluctuations. Typically, the bearing support structure is made of a copper material to take advantage of its high thermal conductivity qualities. However, copper can deform under the significant transient thermal stress that is experienced in the bearing support structure. A deformed rotor bearing support structure causes problems such as hindered and/or unbalanced rotation, resulting in a cathode-anode misalignment. This situation compromises the quality of the x-rays that are emitted from the anode. Moreover, any type of unbalanced rotation results in vibration of the x-ray tube, which increases operating noise of the x-ray device, and ultimately can render the x-ray tube inoperable.
One approach to address some of the problems encountered when using copper as a rotor bearing support material as been use a alternative material, such as stainless steel. However, although stainless steel exhibits better structural rigidity in the presence of high temperatures and thus resists deformation, stainless steel has a lower thermal conductivity. As such, unacceptably high temperatures can be present within the rotor and bearing assembly.
Another significant challenge for heat management in an x-ray device relates to the dissipation of the heat from the x-ray tube to the surrounding structure. Typically, heat is transferred from the x-ray tube to a heat-transfer fluid medium such as a dielectric oil that is disposed within another enclosure, sometimes referred to as an x-ray tube "can" or housing. This housing or can must also exhibit suitable heat transfer characteristics. If the can is not an efficient heat transfer medium, any efficiencies or improvements achieved for heat transfer in the x-ray tube can be neutralized by the can itself.
Typically, the can housing is made of copper or stainless steel. During operation of the x-ray tube, high temperatures are especially prevalent at the window area of the can, which is where the x-ray signals are emitted. Problems can arise in the event that the material that is adjacent to the window does not efficiently draw heat away from the window.
Thus, what is needed in the art is an x-ray tube that can withstand the destructive effects of extreme operating temperatures generated at the rotating target anode. In particular, the x-ray tube components located adjacent to the anode, such as the rotor and rotor skirt, should possess desirable thermal characteristics. Moreover, any solution should reduce or eliminate the occurrence of any foreign debris being released within the evacuated enclosure, such as from flaking or spalling of any coating materials, or from any materials used during the manufacturing process. In addition, it would be an advancement in the art to provide a x-ray tube housing or "can" that is not subject to warpage and structural damage in the presence of high temperatures, and which can efficiently dissipate heat present at the window area.